I have a JD L120 with 200 plus hours on it. I get this vibrating noise from under the seat where the transmission is when the break pedal is not pressed. When I press the break pedal slighty, just barely engaging the break, the vibrating noise disappears. It's loud - almost as loud as the engine when it's on it's lowest throttle setting. So far, performance has not been affected. Any ideas if this can be fixed, or is this just old age?

After doing a search here on AMHD, and with 200hours on it, it does sound like old age.

See:
askmehelpdesk john-deere-l120-transmission- (https://www.askmehelpdesk.com/tools-power-equipment/john-deere-l120-transmission-386741.html)

Look to the right side of where our posts are, and check similar threads.

Hi,
Despite what has been said here and elsewhere about the L120, I have worked on 100s and found all but the first ones to be worthwhile(I have worked on the other brands too.)
The transaxle is easily viewed from the right side slightly to the rear of the tire or from above it. The belt should be in the sheave and the fan intact for proper operation.
My opinion is that you have a bad idler(there are two) and my guess is the V-idler. Bad idlers will slap around and make a ruckus. The idlers were a common failure point and the flat one is Deere part# GX20287 and the V is GX20286.
Picture attached:
Peace,
Clarke

V Idler needed to be replaced. Right you were!